Generally, it is a requirement of a communication system to separate transmit and receive signals so that as much as possible of the transmit band signals are cancelled from the receive band signals.
For example, in an xDSL (x digital subscriber line, where “x” represents a variety of DSL technologies) line interface circuit, a portion of that circuitry known as the “hybrid”, or 2-to-4 wire converter cancels noise in the xDSL line interface circuitry. In some hybrid circuits it may be difficult to achieve enough noise cancellation to meet acceptable performance for the line interface circuit. A reason for this difficulty can be the fact that the transmission line may be limited to sensing the line impedance through the mandatory coupling transformer, and the line coupling transformer has less than ideal properties that alter the perception of the line impedance. The most significance of these properties may be the magnetizing and leakage inductances. To a lesser effect, linearity can be another non-ideal property that alters the perception of the line impedance.
FIG. 1 shows a simplified hybrid circuit. All node voltages are with respect to ground. Vtx+ and Vtx− make up a complimentary differential transmit signal, and Vrx is the receive signal. Trans-hybrid loss, or THL, can be defined as the magnitude of Vrx with respect to Vtx when the line is silent, thus, no receive signal is being received. Vtx can be either the Vtx+ signal or the Vtx− signal since each has the same magnitude.
If T1 was assumed to be an ideal 1:1 transformer and if a matching impedance Zsrc perfectly equaled Zline under all conditions, then a perfect voltage divider is formed, exactly splitting the differential transmit signal in half. Vtx+ and Vtx− can be thought of as plus and minus 1 (unity); therefore, the voltage on Vrx would be zero under these ideal conditions. However, the transformer may have significant magnetizing and leakage inductances and thus cannot be thought of as ideal. Further, the impedance matching source Zsrc may not perfectly equal Zline under all conditions. Thus, a perfect voltage divider under all conditions may not exist for such a hybrid circuit. In fact, the presence of the leakage and magnetizing inductances cause significant degradation of the cancellation, as a function of frequency. Therefore, some prior hybrid circuits suffer limitations including poor cancellation of the transmit signal from the receive signal.
However, for other systems such as very high bit-rate Digital Subscriber Line (VDSL) where there are multiple transmit bands and/or multiple receive bands, there can be a ratio of 400-500 between highest and lowest transmit frequencies. One problem with the conventional approaches, which use a single transformer to couple all bands to the line, is that the leakage inductance of practical transformers limits the high frequency response if the primary inductance is large enough to support the lowest band.